prodromal signs) prior to the emergence of diagnostic characteristics."
It was that sentence from the findings reported by Lori-Ann Sacrey and colleagues  (open-access available here) that caught my eye, and the idea, once again, that movement 'issues' should perhaps be considered a core feature of autism (see here) or at least more of a core feature than they currently are. I might add that I've covered other important findings from Sacrey et al on this blog previously (see here).
Although open-access, a few details might be useful: "Infants were selected at random to comprise three groups of equal size: (1) 10 non-sibling controls (LR [low risk]-control; 7 boys); (2) 10 HR [high-risk] siblings without an ASD diagnosis (i.e., with an older sibling with ASD but did not receive an ASD diagnosis themselves at 36 months; HR-N; 3 boys); and (3) 10 HR siblings with an ASD diagnosis (i.e., with an older sibling with ASD and also received an ASD diagnosis at 36 months; HR-ASD; 6 boys)." As you can see the participant numbers were fairly small but to balance this, authors did report results based on testing "for differences in reaching-to-grasp" across quite a long period (6-36 months) and utilised quite an array of psychometric testing tools pertinent to their population and study aims.
Researchers found that: "Children who were later diagnosed with ASD showed higher (worse) total scores on the reach-to-grasp movement, as well as higher scores on the components of Orient, Lift, and Pronate compared to children in the LR and HR-N groups." They concluded that: "results suggest that such movement mechanics are relevant to monitoring motor development in children at risk for or diagnosed with ASD." Ergo, there may be quite a bit to see when it comes to early surveillance for autism in respect of movement and/or sensory differences.
Just before you leave I'll also bring the findings reported by Emilia Biffi and colleagues  to your attention. Theirs wasn't so much a study of the possible early 'prodromal' signs of autism with a focus on movement patterns but rather another finding suggesting that such movement issues may be persistent in relation to autism and can be readily tested for. Utilising some pretty nifty movement capture technology - "an immersive virtual environment using a 3-D motion analysis system with a dual-belt, instrumented treadmill" - Biffi et al reported that their findings "depicted gait peculiarities in children with ASD, including both kinetic and kinematic features." Movement and gait issues it seems, may very well be a vitally important part of autism...
 Sacrey LR. et al. The reach-to-grasp movement in infants later diagnosed with autism spectrum disorder: a high-risk sibling cohort study. J Neurodev Disord. 2018 Dec 27;10(1):41.
 Biffi E. et al. Gait Pattern and Motor Performance During Discrete Gait Perturbation in Children With Autism Spectrum Disorders. Front Psychol. 2018;9:2530.